To choose a niche, you must find a market and that the people are engaged. Next you need to figure out what they are sharing.

If you need to pick one niche, pick the niche as the most visually appealing and easiest for sharing. Diabetes or any other "cause" may work as a market, but the chances they would share a picture of that niche, versus another niche may be much smaller.

One other point to look at, is some of the most successful niches may have multiple viewpoints. With politics or sports you get passionate people that may not agree with what everyone has to say. That will only drive up traffic and sharing.